It is difficult to separate knocking signals which occur in the operation of an internal combustion engine from time to time--that is, if the engine is placed under knocking operating conditions--from background noise signals which tend to mask the knocking signals and which may have, at least for a short period of time, similar characteristics.
German Patent Disclosure Document No. DE-OS 33 42 466 describes a system to recognize combustion conditions resulting in knocking of the engine. The system includes a comparator and a knocking filter. The knocking filter is connected to the input of the comparator, which may be a comparison network, the other input of which is connected to a reference source. If the output of the knocking filter exceeds the reference level, then a pulse is provided, at the output of the comparison system, which is used to recognize knocking conditions of the operation of the engine.
The knocking filter, generally, is formed as a bandpass filter having a center frequency which is tuned or set to the center frequency of the knocking signal which is to be expected. The output signal of a knock sensor includes not only knocking signals--when they occur--but continuously background noise signals as well as statistically distributed disturbances, which are derived in the course of operation of the internal combustion engine, and are due to its operation. If the reference voltage for the comparison system is derived from the background noise signal--as described in numerous versions of knocking recognition systems and methods, then the statistically distributed disturbances may also lead to exceeding the reference threshold of the comparator system and thus may contribute to an erroneous recognition of operation of the engine under knocking condition. Similarly, knocking recognition circuits which have a controlled input amplification may provide erroneous output signals; in such circuits, the control of the input amplification is generally controlled by a reference voltage which is set to provide a fixed quiescent or base voltage level.
The Invention.
It is an object to improve the separation of knocking signals from the overall signal derived from a knocking sensor, so that knocking conditions of operation of the engine can be reliably recognized by reliably recognizing only knocking signals which occur in the mixture of background noise signals, knocking signals, and statistically distributed disturbance signals, and all applied to a knocking signal recognition system or apparatus.
Briefly, the received signal is correlated with a cyclically varying correlation signal having an average frequency which is a characteristic of a function of the frequency of the knocking signals occurring upon knocking operation of the internal combustion engine (ICE), the cyclically varying correlation signal being derived from the received signal. An output signal is provided if the correlation between the cyclically varying signal and the received signal is established.
The cyclically varying correlation signal may have an average frequency corresponding at least approximately to the knocking frequency of the internal combustion engine; it may respond or include, additionally, whole number multiples of the knocking frequency expected in the derived signal which, likewise, may have these harmonics; in accordance with another feature of the invention, one half of the reciprocal of the knocking frequency expected of the internal combustion engine may be selected as the average frequency of the cyclically varying correlation signal.
In accordance with a feature of the invention, a correlation filter is provided to separate disturbances and noise signals from the knocking signals. The correlation signal suppresses the first pulse, or single individually occurring pulses in the signal derived from the sensor. This permits reliable recognition of engine knocking even if the signal/noise ratio is highly unfavorable.
The system and method of the present invention has the advantage that single, and initial pulses are suppressed and that the apparatus to do so requires only inexpensive and easily derived circuitry components. Further, the method can be carried out entirely by software of a microprocessor.
Knocking signals can be recognized which have a level which is within the overall level of the noise signal. This permits reduction of the comparison threshold level of a comparator with a given reliability of recognition, that is, separation of the knocking signal from the derived signal. The system and method thus permits recognition of knocking signals of lesser intensity than previously possible, resulting in further improvement of engine operation and control thereof close to the knocking limit. The correlation filter can be constructed in form of a microcomputer which, additionally, reduces the requirements for circuit components.